Fluid-operated servo mechanism



Oct. 23, 1951 J. J. RIGHTON I FLUID-OPERATED SERVO MECHANISM Filed March14, 1950 Patented Oct. 23, 1951 FLUID-OPERATED SERVO MECHANISM JosephJohn Righton, Birmingham, England, as-

signor to Joseph Lucas Limited, Birmingham,

England Application March 14, 1950, Serial No. 149,495

l In Great Britain March 23, 1949 4 Claims. 1

This invention relates to fluid-operated servomechanisms of the type inwhich a spring-loaded piston is slidable in a cylinder, and isresponsive to the diiference of fluid-pressure acting on the oppositesides of the piston.

The object the invention is to provide improved means for controllingthe action of a servo-mechanism of the said type.

The invention comprises the combination of a valve adapted to set up apressure difierence in a fluid flowing in a duct, the regions at theopposite sides of the valve being adapted to be connected to theopposite ends of the servo-cylinder, an adjustable throttle throughwhich the said fluid can flow after passing through the valve, and meansresponsive to the pressure difference across the throttle for actuatingthe valve.

In the accompanying drawings:

Figures 1 and 2 illustrate diagrammatically two embodiments of theinvention.

A servo-mechanism of the type aforesaid can be employed for a variety ofpurposes. In particular it can be used as shown in Figure 1 forregulating the output of a variable-delivery pump employed for supplyingliquid fuel to a prime mover or a furnace, or for actuating any devicefor regulating the rate of supply of fuel. For such, or any otheranalogous use, the servo-mechanism comprises a cylinder a, a piston bslidable in the cylinder, and a spring c acting on one side of thepiston, and a rod (1 extending from the piston through one end of thecylinder. In the example shown the said rod serves to impart movement toan angularly adjustable swash plate e with which co-operate the outerends of plungers carried by a rotary pump body 9.

In carrying the invention into efl'ect as shown in Figure 1 there isprovided a hollow body part )1. adapted to be connected to a liquid fueldelivery pipe 2' connected to the pump, and having therein a passage 7'through which the fluid can flow. The passage includes a. seating is fora poppet-type valve m, and at the delivery side of the valve is provideda passage or pipe 11 leading to one end of the servo-cylinder. The otherend of this cylinder is in communication with the delivery passage ofthe pump by way of a passage 0.

Between the valve 1n and the outlet end of the passage 1 is provided aslidable throttle p, the latter being movable by any appropriatemanually or automatically actuated means such as a rack q. pinion r andlever 8. Alternatively it may be responsive to thefluid pressure in anypart of the associated system, or to a thermostat, or a barometricdevice or a centrifugal governor.

2 In association with the valve m there is provided in the body part Ita chamber t containing a deformable capsule u which is loaded by aspring 12 and which at one end is open to the inlet side of the valve,the valve being connected to the closed end of the capsule by a stem w.Also the said chamber t is in communication (by way of a branch passage:r) with the part of the main passage 7' through which the fluid flowsafter it has passed the throttle p. Alternatively the-cap-- sule andspring may be arranged as shown in Figure 2, where the pressure of theliquid in the passage 2 acts upon the exterior of the capsule, and thepressure of the liquid in the passage :1: acts on the interior.

In the example shown in Figure 1, the fluid is a liquid fuel required tobe supplied to a burner indicated by z.

The arrangement is such that the valve m serves to set up a fluidpressure difference in the two regions at the opposite sides of thevalve, which regions are in communication with the ends of theservo-cylinder. The fluid pressure in the anterior region also acts onthe capsule and balances the fluid pressure acting on the anterior sideof the valve. The fluid pressure in the posterior region is variableunder the control of the throttle p. as is also the pressure in thechamber t containing the capsule.

As already indicated, the servo-mechanism is responsive to thedifierence of fluid pressure at the opposite sides of the valve m. Whenthis pressure difference is relatively large (as when the valve isclosed or partially closed) the pressure acting on the Side of thepiston remote from the spring in the servo-cylinder, will move thepiston in opposition to the spring and so effect a reduction of thefluid flow in the system. On the other hand when the pressure difierenceis relatively low (as when the valve is fully open) the spring will movethe piston in the opposite direction for increasing the flow in thesystem. The extent of opening of the valve m depends on the pressurediiierence across the throttle p which is variable as above indicated.Thus, as the pressure difference across the throttle p varies with theposition of the throttle, the fluid pressure acting on the capsule ualso varies correspondingly, and determines the position of the valve mfor any given setting of the throttle. Consequently, the extent ofopening of the valve m, and the pressure difierence across it, also varywith the position of the throttle p, and as the position of the piston bdepends on the pressure difference across the valve. it follows that theoutput of the pump is variable in response to variation in the throttlesetting. With any given open position of the throttle p the fuel flow tothe burner 2 can be maintained constant. By opening the throttle pwider, the pressure difference across the throttle is reduced, causingthe fluid pressure acting on the capsule u to be increased. Consequentlythe valve m is opened further, and the pressure difference across it iscorrespondingly reduced, with the result that the fluid pressuresupplementing the action of the spring on the piston b is increased, andthe latter is moved in the direction for increasing the pump outputuntil a new condition of equilibrium is reached. By moving the throttlep to restrict the fluid flow through the passage the pressure differenceacross the throttle p is increased, causing the fluid pressure acting onthe capsule u to be reduced. Consequently the valve m is moved towardsits closed position, and the pressure difference across it iscorrespondingly increased, with the result that the fluid pressuresupplementing the action of the spring c on the piston b is reduced, andthe latter is moved in the direction for reducing the pump output untila new condition of equilibrium is reached. It will be seen, thereforethat the pump delivers more fuel with increased opening of the throttlep, and less fuel when the throttle is moved in the opposite direction.

Instead of the poppet-type valve shown in Figure l, a piston-type valvemay be used as shown in Figure 2. In this case the valve head m iscontained in a cylindrical and open-ended part 3 having therein lateralports 4. It is convenient though not essential to shape the valve headas shown, so that it presents a sharp peripheral edge to the innersurface of the part 3, but a plain cylindrical head could be used. Theeffective area of the ports 4 is dependent on the position of the head mwhich is variable in the same way as the valve 111. shown in Figure 1.Further, in this figure there is shown the alternative arrangement ofcapsule and spring above mentioned.

By this invention, the control of a servo-mechanism for regulating therate of flow of liquid fuel in a system can be effected in a very simpleand convenient manner. The invention may, however, be applied toservo-mechanisms for other analogous uses, and subordinate details ofconstruction or arrangement of the means constituting the invention maybe varied to suit different requirements.

Having thus described my invention what I claim as new and desire tosecure by Letters Patent is:

l. Mechanism comprising the combination with a. servo-cylinder, aservo-piston slidable in the cylinder under fluid ressure, and a loadinspring whereby the piston is movable in one direction, of a duct throughwhich fluid under pressure can flow, a valve arranged in said duct tocreate a pressure difierence in the fluid flowing therethrough, apassage for conducting pressure fluid from the duct at the outlet sideof the valve to the end of the cylinder away from which the piston ismovable by its loading spring, another passage for conducting pressurefluid from the duct at the inlet side of the valve to the other end ofthe cylinder, an adjustable throttle arranged to control the flow offluid through the duct at the outlet side of the valve, and meansresponsive to the pressure difference across the throttle for actuatingthe valve.

2. In means as claimed in claim 1, the combination with a body partcontaining the valve, of a chamber, a deformable capsule and springcontained in the said chamber, the capsule forming the means responsiveto the pressure diiference across the throttle, and meansinterconnecting the capsule and valve.

3. Means as claimed in claim 1, in which the valve is of thepoppet-type.

4. Means as claimed in claim 1, in which the valve is of thepiston-type.

JOSEPH JOHN RIGHTON.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Number Name Date 2,354,634 Griswold July 25, 1944

